In cryptography, a party will typically possess a set of credentials determining its identity, which tasks it can perform, which protocols it can participate in and so on. These credentials will typically correspond to the party in quest ion having some secret information such as a secret key or another exclusive method of authentication. Yet another credential of the party could be the geographical location of the party.
The geographical location of the party is valuable in a number of settings. For example a first party might want to send a message having some classified information in such a way that it can only be read by a second party present at a specific location. An adversary may try to intercept and decrypt the secret message and so, from the perspective of the first party, it would be desirable to add an additional layer of security that would guarantee that anyone reading the message is physically located at a specific geographic location. In another example, the first party may receive some message purporting to be from a trusted second party situated at a specific geographical location. In order to be sure that the first party has not in fact received a message from a malevolent adversary, it would be desirable for the first party to have a guarantee that the message did indeed originate at the specific geographical location of the trusted second party.
The traditional method of verifying a location of a target is based on calculating a time between a verifying party sending a challenge to the target and receiving a correct reply to the challenge from the target. By utilising multiple spacially-separated verifiers, it is possible to determine a position of the target object by trilateration. However, there are security problems with this traditional approach, as will now be discussed with reference to FIG. 1. FIG. 1 shows an example set-up of a verification system for verifying the location of a target object according to an exemplary prior art system. In this example, the verifying system comprises a first verifier 110, a second verifier 112 and a third verifier 114. The first verifier 110, the second verifier 112 and the third verifier 114 may communicate with each other via a communication channel 116. In order to verify the position of target 120, the first verifier may send a first bit string to the target 120. Similarly, the second verifier may send a second bit string to the target 120 and the third verifier 114 may send a third bit string to the target 120 (see dashed lines). The target 120 may then process the received first, second and third bit strings and send replies to each of the verifiers. On receiving a response from the target 120, each verifier, 110, 112, 114, computes a time between the transmission of their respective bit string and the receipt of the response from the target 120. If each of the verifiers 110, 112 and 114 received an anticipated response within an acceptable timeframe, then the verification system determines that the target object 120 is at the expected location.
However, as can be seen in FIG. 1, an adversary may deploy interceptors between each of the verifiers 110, 112 and 114, and the expected position of the target 120. That is, a first interceptor 130 may be deployed between the first, verifier 110 and the expected position of the target 120, a second interceptor 132 may be deployed between the second verifier 112 and the expected position of target object 120, and a third interceptor 134 may be deployed between the third verifier 114 and the expected position of target object 120. Each of the interceptors 130, 132, 134 is assumed to be in communication with the other interceptors via communication channels 136. In the presence of these interceptors 130, 132, 134, any bit strings sent to the expected position of target 120 from the verifiers 110, 112, 114, may be intercepted by the interceptors 130, 132, 134. Due to the geometry of the setup in FIG. 1, the first, second and third bit strings from the verifiers 110, 112 and 114 can be shared and processed by the interceptors 130, 132 and 134 in sufficient enough time to compute the anticipated response and reply within the acceptable time frame. Accordingly, there is a risk that a response received by the verification system has in fact originated at one or more of the interceptors 130, 132 and 134. If this is the case then the position of the target 120 has not in fact been verified and, indeed, the target 120 may not even be located at the expected position. There could therefore be security implications for future messages sent to the expected position of the target object.
The present disclosure aims to overcome at least some of the problems described above by providing a truly secure method of verifying the position of a target. The invention described herein is, at least in part, based on quantum mechanics, quantum information and quantum computation. For the interested reader, the fundamentals of these fields are detailed in “Quantum Computation and Quantum Information” by Michael A Nielsen and Isaac L Chuang. In particular, this reference contains information concerning properties of qubits and the basics of quantum measurements in complementary bases. This reference also familiarises readers with notations conventionally used in the field of quantum physics.